A simple air operated reciprocating piston pump of the type such as typified by Graco's FAST-FLO.RTM. pump have been known for many years. Such pumps perform an excellent job at their intended function namely the pumping for transfer purposes of basic fluids such as paints, oils and the like. Occasionally though, such pumps may be reluctant, particularly at low cycle rates, to change over from one stroke to the other.